In mobile radio telephone systems, the capability is provided to transfer the handling of communications with a mobile station from one base station to another, as the mobile station changes its position and so moves out of the coverage range of one base station and into the coverage area of another base station. This process is commonly termed handover or handoff.
To smoothly complete a handover, the network controlling the base stations must first determine, for each mobile station, whether the need for handover is imminent and secondly determine to which new base station handover should be effected. In making the latter decision it is desirable that the network controller know either how well each base station can receive signals from a mobile station in question, or how well the mobile station in question can receive signals from each base station, or both. The present invention provides a method for the base station to acquire this information using a code division multiple access (CDMA) transmission method.
Conventional mobile telephone systems were based largely on Frequency Division Multiple Access (FDMA), in which each mobile station transmits on a unique frequency within its current base station area. The mobile station is thus unaware of signals on other frequencies from surrounding bases. In FDMA systems it would be too costly to equip mobile stations with an extra receiver that could be used to scan other base frequencies. Instead, it is established practice that base stations are equipped with a scanning receiver that looks out for the signals of approaching mobile stations. The network then hands over a mobile from a base station covering an area it is leaving to the base station that reports the best reception of the mobile station's signal.
More recent cellular telephone standards employ Time Division Multiple Access (TDMA) in which a fixed time period (e.g., 20 mS) on each radio frequency is divided into a number (e.g., 3) of short timeslots (e.g., 6.6 mS) that are cyclically used by different mobile stations. Thus, a first mobile station transmits in the first timeslot in each period, a second mobile station transmits in the second timeslot in each period and so on. Likewise the base station transmits to one mobile station in the first timeslot, another mobile station in the second slot and so on. By offsetting the allocation of timeslots in the two communications directions, base to mobile (the downlink) and mobile to base (the uplink), it can be arranged that a first mobile transmits in the first timeslot and receives in the second timeslot; a second mobile transmits in the second timeslot and receives in the third, while a third mobile transmits in the third timeslot and receives in the first timeslot. An advantage of this arrangement is that a mobile station does not need to transmit and receive simultaneously, which facilitates sharing a single antenna.
In the above three-timeslot example, each mobile station is active to transmit or receive in two of the three timeslots and idle in the remaining timeslot. Therefore it is possible for TDMA mobile stations to use this idle time to search for signals from other base stations and measure their signal strength. By reporting these signal strength measurements to the base station using a slow speed data channel multiplexed with the traffic (i.e., voice), the network is informed about the base stations each mobile station can receive. The network can use this information to effect handover to the best base station, and such a system is termed mobile assisted handover (MAHO). When the base stations scan for the signal strength of mobile stations, the system could be termed base assisted handover (BAHO).
Systems providing MAHO also have access to the base station measurements, and so are able to effect smoother and more reliable handovers because both uplink and downlink signal strengths are taken into account, instead of just uplink strengths in the case of BAHO. However, these conventional systems have a number of limitations. For example, MAHO has conventionally only been used in TDMA systems. TDMA systems, however, involve a certain waste of capacity due to the need for guard spaces between timeslots during which the mobile stations' transmitters power up and down. Moreover, in these TDMA systems with MAHO, fast frequency switching is needed to scan channels on which other base stations are transmitting during the short idle periods, which is technically difficult and adds both complexity and cost to the system. The available time in the idle slot combined with the difficulty in switching frequency rapidly permit only one neighboring base frequency to be scanned per 20 mS frame. In FDMA systems, MAHO has not been implemented because base stations in FDMA systems use different frequencies to transmit control channels than those being used by mobile stations for transmissions and FDMA mobiles cannot change frequency without loss of traffic.
The present invention uses Code Division Multiple Access (CDMA) to permit neighboring base stations to share the same frequency channel, and thus permit the mobile to assess their signal strength without changing frequency or losing traffic. Another feature of the present invention is that the signals generated by the neighboring base station to which the mobile station is being handed over comprise a diversity transmission which can be combined with the transmissions from the base station originally connected to the mobile station to improve reception quality.
Conventionally, such diversity transmissions have been provided in CDMA systems by transmitting a signal which is encoded and modulated in exactly the same way as the original signal with a relative delay of one or more chips or bits. These overlapping signals can then be combined in an echo-integrating type of receiver such as a Viterbi equaliser or a RAKE receiver.
A disadvantage of these conventional macrodiversity systems is the need to transmit from one base station the codes that are being used to transmit to the mobile station to one or more other base stations. In a subtractive CDMA system, this also involves informing all of the mobile stations in a cell when a diversity transmission for any mobile is initiated of the exact code that will be used.
This problem is overcome according to the present invention by using different codes for the diversity transmissions so that an originally connected base station need not transmit an allocated code to other base stations. This takes advantage of the fact that a CDMA mobile receiver is able to simultaneously receive and decode both its normally coded signal and the diversity coded signal.